The present invention relates to a method of navigating an agricultural vehicle, said vehicle being equipped with a 3D imaging device for imaging surroundings of the vehicle, and an image processing device.
Within agriculture it is a wish to be able to measure or establish the speed, the position, the heading etc. of a vehicle in order to be able to navigate the vehicle and any implement attached thereto in an agricultural field.
Methods and devices currently used in navigating agricultural vehicles comprise:
Wheel odometry, which uses encoders to measure how many revolutions the wheels have turned. Wheel odometry is sensitive to tire pressure and wheel slip. Wheel odometry does not give an absolute position and it drifts over time. The turn angles for the vehicle are hard to measure exactly due to tire slip of the steering wheels.
Radar, which can be used for velocity estimates, and which does not suffer from problems of wheel slip. It is however less accurate due to the physical ways it measures signals. Radar is therefore not very usable for accurate positioning.
Global positioning system (GPS), which depending on the type of sensor used (ranging from cheap low accuracy to expensive high accuracy systems) can give a full 3D position estimate. One drawback of the GPS system is that it requires an unobstructed view to the GPS satellites. Typically this cannot always be guaranteed meaning the system has a tendency to fail at times. There are many sources of errors in GPS including multi-path errors and atmospheric effects. Typical situations for agricultural use where GPS does not work very well includes: near structures or trees (e.g. in the headland of a field), and on hilly terrain.
Inertial navigation systems (INS) comprising an Inertial measuring unit (IMU) with 3 accelerometers and 3 gyros that measure accelerations and turn rates on all three spatial axes (6 degrees of freedom).
The most accurate systems commercially available comprise GPS and IMU sensors for positioning. These are commonly referred to as GPS aided inertial navigation systems (INS). The problem with a system of this kind is that position estimates quickly drift if the GPS signal is lost. Also it has a long settling time before it runs stable.
US 2004/0221790 A1 discloses a method and an apparatus for optical odometry, which may be implemented e.g. in a farm vehicle, such as a tractor, in a shopping cart or in an automotive anti-locking braking system. The apparatus comprises an electronic imager such as a monocular camera, which is directed vertically downwards to image the ground below the vehicle, which thereby works in the same way as an optical computer mouse. Combination with GPS is suggested.
US 2007/0288141 A1 discloses a method for visual odometry using in one embodiment a stereo camera. The method is suggested to be implemented in an object requiring navigation such as an autonomous (e.g. unmanned) vehicle or in a robot. Navigation in difficult or dangerous terrain without putting human operators at risk is mentioned. The fields of use envisaged in this publication are thus remote from agriculture. It is suggested to supplement the method of visual odometry with data from other sensors, such as GPS, inertial or mechanical sensors. According to the method a sequence of image frames are received and analysed to establish a change of pose and thus a new pose of the camera providing the image frames. According to the method feature points are found in a first image frame and they are then tracked in subsequent image frames e.g. for as long as they are in the field of view. Based on the sequence of image frames incremental pose estimates are established iteratively. When a criterion is fulfilled the iteration is stopped and a firewall is inserted whereby future triangulations of 3D points will not be performed using observations that precede the most resent firewall. Thus this method so to speak starts from a firewall and calculates forward a change of pose until a new firewall. Thus valuable information about change of pose immediately prior to the most resent firewall is lost for future estimates. This is adversely to the accuracy of the method and the reliability of its results.
WO 2007/031093 A1 discloses an agricultural vehicle equipped with a 3D imaging device imaging a part of the ground surrounding the vehicle in front of it. The 3D imaging device is used for tracking a swath on the ground with a view of picking up the swath by means of a baler.